Re: [RFC bpf-next v1 2/8] bpf: no_caller_saved_registers attribute for helper calls

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-07-03 at 11:57 +0000, Puranjay Mohan wrote:

[...]

> > +static u32 call_csr_mask(struct bpf_verifier_env *env, struct bpf_insn *insn)
> > +{
> > +	const struct bpf_func_proto *fn;
> > +
> > +	if (bpf_helper_call(insn) &&
> > +	    verifier_inlines_helper_call(env, insn->imm) &&
> 
> This should also check bpf_jit_inlines_helper_call(insn->imm) as the JIT
> can also inline helper calls separately from the verifier.
> 
>     if (bpf_helper_call(insn) &&
>         (verifier_inlines_helper_call(env, insn->imm) || bpf_jit_inlines_helper_call(insn->imm)) &&
> 
> This is currently being done by the arm64 and risc-v JITs and they don't
> scratch any register except R0 (The helpers inlined by these JITs are
> non-void).

Hello Puranjay, thank you for commenting.
In a sibling email Andrii suggested to also add a function like below:

    __weak bool bpf_jit_supports_helper_nocsr(s32)

At the moment I see the following helpers inlined by jits:
- arm64:
  - BPF_FUNC_get_smp_processor_id
  - BPF_FUNC_get_current_task
  - BPF_FUNC_get_current_task_btf
- riscv:
  - BPF_FUNC_get_smp_processor_id

I suspect (but need to double check) that all of these patches conform
to nocsr. If so, what are you thoughts regarding bpf_jit_supports_helper_nocsr():
do we need it, or should inlining imply nocsr?

Thanks,
Eduard